1. Field
The present invention relates to a lithographic apparatus and a method for manufacturing a device.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs), micro-electro-mechanical-systems (MEMS), and other devices involving fine structures. In a conventional apparatus, a contrast device or a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display or other device. This pattern can be transferred onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a glass plate). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can comprise a patterning array that comprises an array of individually controllable elements. Compared to mask-based system, the pattern can be changed more quickly and for less cost.
In general, a flat panel display substrate is rectangular in shape. Known lithographic apparatus designed to expose a substrate of this type typically provide an exposure region, which covers a full width of the rectangular substrate, or which covers a portion of the width (e.g., about half of the width). The substrate is scanned underneath the exposure region, while the mask or reticle is synchronously scanned through the beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate, then exposure is completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate is moved transversely after the first scan, and a second scan is performed to expose the remainder of the substrate.
Another way of imaging includes pixel grid imaging, in which a pattern is realized by successive exposure of spots.
When manufacturing devices using a lithographic process, more than one device can be formed on a substrate. For instance, the same patterns can be exposed at different locations on the substrate to form the features of each device. Subsequently, following the processing to form the devices, the substrate is divided to form the separate devices. In mask-based lithography, an area exposed on the substrate at each area is defined by the mask and therefore fixed. Accordingly, subject to variations caused by processing differences, the patterns exposed on each part of the substrate, and hence the features of each of the devices, are substantially identical for each of the devices formed on a substrate.
When performing a lithographic process using an apparatus that has an array of individually controllable elements, it is possible to expose different patterns on different portions of the substrate. However, the cost of a lithographic apparatus using an array of individually controllable elements can be significantly reduced if it is configured to expose an identical pattern in a plurality of areas on a substrate.
In a particular arrangement, a repeating pattern that is to be exposed on a substrate (which can correspond to the pattern to form a layer of the device to be formed on the substrate) can be divided into a plurality of stripes. During the exposure process, the substrate is scanned beneath the beam of radiation, which is patterned by the array of individually controllable elements to expose one of these pattern stripes in a first area on the substrate. The same stripe is then printed on each of the other areas on the substrate at which the device is to be formed. Once the pattern stripe has been printed at each location, the lithographic apparatus begins printing the next pattern stripe at each location. The benefit of such a system is that it reduces the cost of the processing hardware that is required to control the individually controllable elements in the patterning array.
In one example, the amount of data to define the pattern to be exposed on the substrate can be very large. Accordingly, it is conventionally stored in a vector-based format, which permits a considerable compression ratio compared to a bitmap format. In order to calculate the control signals to drive the array of individually controllable elements, the pattern data is converted into a format, such as a bitmap data format, and stored in a data buffer. This process, commonly referred to as ‘rasterization,’ is complicated and the rasterizer represents a significant portion of the cost of the lithographic apparatus. Accordingly, it is desirable to minimize the cost of the rasterizer. In one example, this can be achieved by selecting a rasterizer size (and hence speed at completing the rasterization process) such that it can just rasterize the data for the next stripe of the pattern to be printed during the time taken to print the preceding stripe on each of the areas on the substrate that the pattern is required. For example, if twenty identical devices are to be formed on one substrate, the rasterizer can be twenty times slower than it would be if each of the devices on the substrate were to be different. This is because the time provided for the rasterizer to rasterize the next stripe is the time taken to print twenty identical stripes on the substrate rather than the time taken to print a single stripe. The use of a slower rasterizer means that the rasterizer is cheaper, resulting in the cost savings of the lithographic apparatus referred to above. However, in order to achieve this cost saving, the apparatus is limited to providing a plurality of identical patterns on the substrate.
Therefore, what is needed is a system and method which are not limited to providing multiple identical versions of a same pattern on a substrate without significantly increasing the cost of the apparatus.